Synthetic cis-1,4-polyisoprene possesses properties that are similar to natural rubber in many respects. It can accordingly be used beneficially in a wide variety of applications, such as in tires and in a wide variety of other rubber products. For instance, synthetic cis-1,4-polyisoprene is widely used in manufacturing rubber thread that is used in making elastic bands and in articles of clothing.
U.S. Pat. No. 3,386,983 discloses a process for producing polyisoprene having predominantly 1,2-cis-linkages which comprises solution polymerizing isoprene monomer in a hydrocarbon solvent selected from the group consisting of aliphatic, cycloaliphatic and aromatic hydrocarbons with an organometallic mixed catalyst composed of a trialkyl aluminum compound and titanium tetrachloride, wherein said polymerization is conducted in the presence of 0.05 to 100 moles of a polyhalogenated lower aliphatic or cycloaliphatic hydrocarbon per mole of titanium tetrachloride.
British Patent 1,186,490 discloses a process for the production of polyisoprene of predominantly 1,4-cis-structure, wherein isoprene is polymerized in the presence of a polymerization catalyst that is comprised of a trialkylaluminum compound, titanium tetrachloride and an ether of the general formula R--O--R in which R represents an alkyl radical containing from 3 to 6 carbon atoms, wherein the molar ratio of the trialkylaluminum compound to ether is within the range of 1:0.3 to 1:0.6 and wherein the molar ratio of the trialkylaluminum compound to titanium tetrachloride is within the range of 0.75:1 to 1.2:1.
Australian Patent 582,273 discloses a process for continuously producing a preformed catalyst system that is useful for catalyzing the polymerization of isoprene into polyisoprene. The process of Australian patent 582,273 involves mixing titanium tetrachloride with a mixture of a trialkyl aluminum compound and an ether in a first reaction zone to produce a catalyst slurry which is continuously withdrawn from the first reaction zone and added to a second reaction zone for aging to form the preformed catalyst.
The synthesis of cis-1,4-polyisoprene by polymerizing isoprene with a catalyst system which is comprised of a titanium tetrahalide, a trialkylaluminum compound and diphenylether is plagued with the formation of gel. In fact, cis-1,4-polyisoprene that is synthesized with such catalyst systems can contain 45 percent or more gel. In many applications, high levels of gel are undesirable or unacceptable.